Problem: Luis ate 5 slices of cake. Christopher ate 5 slices. If Luis ate $\dfrac{5}{13}$ of the cake, what fraction of the cake is remaining?
Solution: If 5 slices represent $\dfrac{5}{13}$ of the cake, there must have been a total of 13 slices. $\text{fraction of cake remaining} = \dfrac{\text{number of slices remaining}}{\text{number of slices total}}$ Together they ate 10 slices, which leaves 3 out of 13 slices remaining. There is $\dfrac{3}{13}$ of the cake remaining.